This invention relates, in general, to solid state sensor devices, and more particularly, to solid state flow sensor devices.
Flow measurements of fluids (gases and liquids) are critical in many application areas including biomedical, automotive, aerospace, chemical, and heating, ventilation, and air conditioning (HVAC) applications. Several techniques are known for measuring the flow velocity of fluids.
Hot wire anemometers are often used to measure or sense mass air flow. Hot wire anemometers typically consist of an electrically heated fine wire (usually platinum) exposed to a gas stream whose velocity is being measured. An increase in fluid velocity, other things being equal, increases the rate of heat flow from the wire to the gas, thereby tending to cool the wire and alter its resistance. In a constant-current anemometer, gas velocity is determined by measuring the resulting wire resistance. In a constant-resistance type anemometer, gas velocity is determined from the current required to maintain the wire temperature, and thus the resistance constant. The hot wire anemometer can be modified for liquid measurements, although problems arise from bubbles and dirt adhering to the wire. Hot wire anemometers are big, expensive, require expensive monitoring and/or control equipment, and are limited in their areas of use because the wire is exposed to the fluid under measurement.
A micro-machined silicon mass air flow sensor version of the hot wire anemometer also has been reported. This device uses temperature resistive films suspended over a micro-machined cavity. Typically, heating and sensing resistors are formed in a cross pattern. Heat is transferred from one resistor to the other as a result of air flow. The imbalance in resistance caused by the heat transfer is directly proportional to the flow velocity. The micro-machined version of the hot wire anemometer is expensive, difficult to manufacture, and still requires expensive monitoring and/or control equipment.
Other known approaches to flow velocity measurements include venturi-type flow meters. Venturi-type flow meters operate on the principle that when a constriction is placed in a closed channel carrying a stream of fluid, an increase in velocity, and hence an increase in kinetic energy, occurs at the point of constriction. Based on an energy balance relationship, this increase in velocity results in an decrease in pressure. Flow velocity is obtained from, among other factors, the decrease in pressure, the cross-sectional area of the constriction, and the density of the fluid. Prior art venturi-type flow measurement devices are very bulky and heavy and are not suitable for applications where space is a premium and excessive weight is a concern. Also, prior art venturi-type devices require expensive monitoring and/or control equipment.
Accordingly, a need exists for a flow sensor that is suitable for use in many environments, that is cost-effective, that reduces external monitoring and/or control equipment requirements, that utilizes existing pressure sensing technology, and that is small and lightweight.